Injection Moulding — design guidelines
Moulding rules are different in kind from machining rules: the part has to fill, cool evenly and then come out of the tool. Nearly every moulding defect — sink, warp, short shot, drag mark — traces back to a wall thickness or a draft angle decided in CAD.
Constraints specific to this process
These are the limits that come from this process in particular. The general rules on wall thickness, corner radii and hole depth apply on top of them.
| Parameter | Design to thisNo surcharge, no discussion needed | We can reachAchievable, but affects price or lead time | Where the limit comes from |
|---|---|---|---|
| Draft angle on a vertical wall | 1.5° per side | 0.5° polished, 3° textured | Without draft the part grips the core and drags as it ejects, leaving scuff marks. Textured surfaces need more draft because the texture itself is an undercut. |
| Nominal wall thickness | 2.0 – 2.5 mm | 0.8 – 4.0 mm | Thick sections cool from the outside in and shrink away from the surface, which is what a sink mark is. Thin sections may not fill at the far end of the flow path. |
| Rib thickness vs wall | 0.5 × wall | 0.6 × wall | A rib thicker than 60 % of the wall creates a thick spot at the junction, which shows as a sink mark on the opposite cosmetic face. |
| Tolerance on a moulded dimension | ±0.1 mm or ±0.3 % | ±0.05 mm | Shrinkage varies with melt temperature, hold pressure and how long the part sits. Below ±0.05 mm the feature has to be machined after moulding. |
| Break-even against machining | 2,000+ pcs | 500 pcs with aluminium tool | The tool is the cost. A soft aluminium mould brings the break-even down to a few hundred pieces but wears out after a few thousand shots. |
What we see go wrong most often
Recurring issues on the drawings that reach us. None of them are hard to avoid once you know to look for them.
- Sending a part designed for machining and asking for it to be moulded. Sharp internal corners, zero draft and uniform solid sections all have to be reworked first — we do that as part of the quotation.
- Not saying which faces are cosmetic. Gate location, ejector marks and the parting line all have to go somewhere — if you do not choose, we do.
General machining rules
The numbers that apply across every cutting process — wall thickness, radii, holes, threads, tolerances and cost drivers.
- Wall thickness
- Internal corner radii
- Holes and threads
- Pockets, cavities and undercuts
- Tolerances, text and surface callouts
- What actually drives the price
Every quotation includes a manufacturability review against these rules. If something in your model falls outside them, we tell you before you commit, not after the first article fails.
Not sure whether your part clears these limits?
Send the STEP file. A manufacturing engineer marks up anything that will be difficult and returns it with the quotation — no charge, no obligation.